Hello,
I am on an o2 contract and you can definatley check your remaining tetxs and minutes online its long winded though.
This is how to do it (im rubbish at instructions so I apologise now) once you have signed in it will say my 02 on the top left click on my account and online services, then click on online bill under the view my bill section, lots of links will appear on the left click on the unbilled summary link, there will be a heading saying savings and rewards underneath that is a click thats says inclusive allowance details if you click that link it will allow you to select your mobile number click on your number and it will open up and show you all the texts and minutes you have left.
you can also view an itemised bill of every text and call you have made as well.
hope that helps. Clare